Both resolvers and synchros are well known and widely used devices for measuring angular position (see for instance, "Synchro Conversion Handbook " ILC Data Device Corporation, 105 Wilbur Place, Bohemia, N.Y. 11716. Fourth printing, 1985. Library of Congress Catalog Number 74-77038). When coupled with the use of rack and pinion or belt drives, such devices can also measure linear displacement. The common principle used in these devices is that both a movable and a static member, e.g., a stator and rotor, are equipped with electromagnetic poles and associated windings. The relative position of the two members is determined from the relative amplitudes of the signals generated in the windings of the static member caused by driving the winding of the moving member with a sinusoidal electrical signal. All the signals involved have the same electrical phase. However, it is important to note that because the signal information is encoded in the relative amplitude of the two stator signals, relating this information to the desired angle (.theta.) is a complex process. In addition, it is necessary to employ windings on both the rotor and the stator elements.
Other resolvers are known in which linear displacement or angular position are detected from the phase difference between an output signal and an applied reference signal. For example, U.S. Pat. No. 4,556,886 shows such a linear displacement detector and U.S. Pat. No. 4,754,220 shows such an angular position detector. In both these examples, the output signal is derived from a separate winding on the stationary element of the resolver.
An advantage of detecting position from a phase difference is that the amplitude of the signal involved is irrelevant and the circuitry needed to convert a phase difference to a digital signal is more straightforward than that needed to compare signals with different amplitudes. Accordingly, it is an object of the invention to detect linear displacement or angular position as a phase difference using a minimum number of windings on only one element of a resolver or equivalent structure.
It is often desirable to be able to detect linear or angular position in two dimensions. Accordingly, it is another object of the invention to detect two-dimensional position from phase differences, again using a minimum number of windings on only one element of a resolver or equivalent structure.